The present invention relates to molecular beam epitaxy (MBE) systems.
In molecular beam epitaxy, thin film deposition is achieved by directing molecular beams onto a substrate in an ultra high vacuum. Preferably the beams are not ionized, but are neutral molecular or atomic species, whence the name of the process. The substrate is heated to a temperature where kT is large enough to permit a deposited atom to move laterally for an average distance of at least several angstroms, to permit the deposited atoms to find their energetically preferred sites. Thus, MBE permits growth of thin films with extremely high crystalline quality. The MBE technique is generally well known, and has been widely discussed. See, for example, the following review articles, which are hereby incorporated by reference: A. Y. Cho and J. R. Arthur, in Progress in Solid State Chemistry, edited by J. McCaldin and G. Somorjai (Pergamon, New York, 1975), Vol. 10, p. 157; L. L. Chang, in Handbook on Semiconductors, edited by S. P. Keller (North-Holland, Amsterdam, 1980), Vol. 3 Chapter 9; C. E. C. Wood, in Physics of Thin Films, edited by C. Haff and M. Frankcombe (Academic, New York, 1980), Vol. 11, p. 35; C. T. Foxon and B. A. Joyce, in Current Topics in Materials Science, edited by E. Kaldis (North-Holland, Amsterdam, 1981), Vol. 7, Chapter 1.
Molecular beam epitaxy is very attractive as a product technology for many applications, due to its unique capabilities. MBE easily produces hetero-epitaxial structures, wherein an epitaxial layer of one material is epitaxially deposited onto an underlying layer of a different material. The abrupt epitaxial transitions which can thus be achieved can be rapidly alternated to achieve superlattice structures, wherein, as the different epitaxial layers become very thin, some anomalous and highly desirable properties appear. Such structures are very difficult to make in any other way. MBE can also be used to make strained superlattices, wherein materials which are not lattice-matched in isolation are nevertheless grown in a perfect epitaxial structure. That is, materials which have the same crystal structure, but which would not have the same lattice spacing normally, can not be grown epitaxially by conventional methods. For example, the lattice constant of InAs .sub.0.4 Sb .sub.0.6 is 0.4% less than that of InAs.sub.0.27 Sb.sub.0.73. Thus, if one attempts to grow an epitaxial layer of In As.sub.0.4 Sb.sub.0.6 on a In As.sub.0.27 Sb.sub.0.73 substrate by conventional methods such as chemical vapor deposition, the two lattices would not be matched. That is, it is desirable to have the interface between the two materials preserve the crystalline structure of the materials, so that the first lattice is a smooth continuation of the second lattice, except that more arsenic atoms and fewer antimony atoms are now found on the Group V sites. This can not be achieved by conventional methods, but is readily achieved in superlattice structures by MBE. MBE also promises other unique capabilities, such as epitaxial deposition of insulators over semiconductors, metals over insulators, etc.
However, attractive as these capabilities are for semiconductor device fabrication, MBE systems at present are primarily a laboratory tool rather than a production tool, simply because the throughput of MBE systems is slow. In part, the slow throughput of MBE systems is unavoidable, since it is difficult to achieve good quality deposited material if the deposition rates used are greater than several microns per hour. However, in large part this problem of slow throughput has been due to the difficulties of wafer handling.
Thus, it is an object of the present invention to provide an MBE system having reduced time requirements for wafer handling.
The prior art of MBE systems has used molybdenum substrate holders, with each wafer being soldered to the substrate holder. Due to the extremely high vacuums which are used for growth (typically in the neighborhood of 10-11 Torr), the materials requirements for anything which will be present in the vacuum chamber are extremely stringent. These requirements are especially stringent for items which are in contact with the wafer or are in close proximity to the wafer since contaminants released by these items are particularly likely to be transported to the growth front and be incorporated into the epitaxial layer.
It should be noted that the actual position of the wafer during MBE growth is downward facing. This is done because, even in an ultra high vacuum system, some particulates will inevitably be present, e.g. particulates which adhere to the inner walls of the chamber until released by thermal cycling, accoustic vibrations, or minor mechanical shocks, and flaky arsenic particles from inadvertent arsenic deposition on the cryoshield. At extremely low pressures these particles will not be suspended in the chamber atmosphere, but will simply fall down to the bottom of the chamber. The wafer is positioned facing downward so that it does not intercept the path of any of these falling particulates, and to permit the wafer to face liquid sources such as a liquid gallium source.
It is also well known in the MBE art that the wafer should be rotated during growth. This provides more uniform deposition and heating. In particular, since an MBE system should have multiple molecular beam sources, to take advantage of the flexibility in deposition discussed above, the molecular beams from most of these sources will not impinge on the wafer at right angles to its surface. Essentially all of the depositions are angle depositions, and rotation of the wafer accordingly improves uniformity.
However, the combination of requirements just discussed adds up to a quite difficult mechanical problem in supporting the wafer during growth. Arc-cast molybdenum is an adequately high-purity material for the substrate holder, and can be machined into complex shapes as desired, but unfortunately the thermal expansion coefficient of molybdenum is very different from that of desirable substrate materials, such as gallium arsenide or other III-V semiconductor compounds. This means that the substrate must be held in such a manner that the substrate can expand freely during the temperature rise to the growth temperature, and not be strained by the different thermal expansion of the substrate holder.
The conventional method of the prior art to accomplish these objectives is indium soldering to a molybdenum substrate holder. That is, a very pure molybdenum substrate holder is used, which includes pins (or other mechanical elements) for attachment to the rotating drive in the growth chamber, and the substrate wafer upon which MBE is to be performed is hand-soldered to the substrate holder. That is, the substrate holder is heated to about 150.degree. C. on a hot plate, and the wafer is manually placed upon it. At this temperature the indium will be liquid, but will freeze again to provide a good mechanical attachment as the wafer is cooled down. The wafer, mounted to a substrate holder, is then loaded into the MBE system. After initial pump down stages as will be discussed below, the wafer and substrate holder are eventually transferred into the growth chamber. In the growth chamber, the wafer is held in a downward-facing position, and heated up to the growth temperature. As the wafer temperature increases above the melting point of indium (about 142.degree. C.), the indium melts, but the wafer is still held in position by the good adhesive properties of the liquid indium. After the growth run is complete, the substrate holder is allowed to cool, so that the indium refreezes and the substrate holder and wafer are then transferred back out of the growth chamber. After the wafer is removed from the MBE system, it is necessary to unsolder it from the substrate holder. This is a very delicate manual step. The same viscosity and adhesion which permit indium to hold the wafer in place during the growth cycle make removal of the wafer from the substrate holder difficult. A further difficulty is that the indium distribution will typically change during growth, especially if the molybdenum holder has been used many times, so that the thickness of the indium will often be found to be thicker under the edges of the wafer than under the center of the wafer. After the wafer is removed from the substrate holder, it will typically be found that the indium has partially alloyed with the backside of the GaAs wafer. To avoid producing crystal dislocations the alloyed material must be removed. This is typically done by etching in HCe. To avoid introduction of mobile impurities into the front surface of the wafer during this backside etch, the front surface is typically painted with photoresist prior to this etching step. (Likewise, the indium on the front side of the molybdenum substrate holder can be removed by etching the block in HCl.) However, repeated use of this cleanup etch will gradually erode the molybdenum so that the surface of the substrate holder is no longer planar.
After the indium is etched away, the backside of the wafer is no longer flat. If the wafer is sought to be processed in this condition, very many wafers will be broken during routine handling, since the etched backside will now contain irregularities which can produce local stress maxima. Moreover, use of vacuum chucks in subsequent wafer processing operations now becomes impractical, because the backside of the wafer is no longer flat.
This difficulty can be mitigated by repolishing the backside of the wafer until it is flat again. However, this provides a further manual processing step, in which further wafer breakage is inherently likely. Moreover, if the wafers are not initially of extra thickness, this polishing step will thin them so that they are very fragile and again liable to breakage in routine handling.
Moreover, the front side of the wafer, which now contains the expensive epitaxial layer, must be carefully protected from this further polishing step. A further difficulty with this prior art method is the smoothness of the molybdenum substrate holder. That is, the substrate holder as received (or as fabricated) will not be perfectly smooth, and the irregularities in the substrate holder surface may themselves make removal of the wafer more difficult. This problem can be avoided by polishing the molybdenum substrate holder, but the problem with this is that repeated polishing of the substrate holder will erode it.
All of these extra processing steps, required by the indium soldering, are very expensive. In particular, any processing step which risks slice breakage after the MBE operation has been performed is extremely expensive, since the structure which is thus exposed to a risk of being destroyed may be a structure which has just taken several hours of growth time on a million dollar machine, plus the initial slice cost and the pre-growth slice preparation cost.
Luscher, U.S. Pat. No. 4,201,152, contains a general description of MBE substrate holder manipulation, including transfer of a holder from a movable carriage to a treating station where deposition occurs. The patented invention relates to the contacts of the thermocouple monitoring the temperature of the holder, and no description of the mounting of the substrate on the holder is given. Rather, Luscher notes in passing that the substrate to be coated by the MBE process is mounted on the face of the holder in a known manner.
Salt, Jr., U.S. Pat. No. 4,344,383, illustrates retainer ring springs and complementary substrate holder recesses for holding substrates in the form of circular wafers during low pressure deposition processing. The wafers are simply inserted into the recesses and the retainer ring springs compressed, inserted into the recesses on top of and in contact with the wafers, and released to engage the recess sidewalls. The springs are still under slight compression and therefore do not slip on the sidewalls; further, the sidewalls may be tapered to help prevent slippage of the springs on the sidewalls.
It is an object of the present invention to provide an MBE system having a wafer support structure such that the number of handling steps required after growth is minimized.
It is a further object of the present invention to provide an MBE system wherein no unsoldering step is necessary after the growth step.
To achieve these and other objects, the present invention provides an MBE System wherein the wafer on which epitaxial deposition is to occur is not soldered to a substrate holder. Instead, a substrate holder with a lip approximately as high as the thickness of the wafer is used, and a retaining ring attaches to the substrate holder to hold the wafer in place during the growth cycle. The retaining ring, like the substrate holder, is made of high-purity refractory material, such as arccast molybdenum. The substrate holder and retaining ring are dimensioned to hold the wafer somewhat loosely, to allow for thermal expansion during the cycling up to growth temperature, which is typically about 600.degree. C.
According to the present invention there is provided:
A molecular beam epitaxy system, comprising:
A growth chamber containing a plurality of effusion sources and a substrate support;
means for evacuating said growth chamber to provide an ultrahigh vacuum therein;
a substrate holder and retaining ring, said substrate holder and retaining ring being mutually attachable and dimensioned to hold a wafer of predetermined size there between, said substrate holder being attachable to a substrate support within said growth chamber; and
means for transferring wafers in and out of said growth chamber.
According to the present invention there is provided:
A substrate holder assembly comprising:
a substrate holder and retaining ring, said substrate holder and retaining ring being dimensioned to hold a wafer of predetermined size therebetween, said substrate holder comprising means for attaching said substrate holder to a downward-facing substrate support;
wherein said substrate holder and said retaining ring are mutually attachable and define a cavity therebetween which is slightly larger than said wafer.